Draft: Update behavior of new note split button

What does this MR do and why?

Updates the behavior of the split button for adding a new comment so that clicking on the dropdown options takes immediate action instead of changing the configuration of the button to the left of the dropdown.

This MR was spurred on by a discussion in Slack about the expected pattern for split buttons that perform an action.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Scenario	Before	After
opening the dropdown when there is comment text entered. In the before state, this is a selector and the first item is selected by default. In the after, these items are each a button, and no item is selected or selectable.
Creating a new note while a review draft is in progress on an MR. Note that now there are two separate buttons for Add comment now and Add thread now. This is because the dropdown in the before state controlled the configuration of the Add ___ now button, but since that is no longer a picker but a menu of buttons, this change was necessary to preserve "start a new thread immediately while a review is in progress" functionality for MRs.

How to set up and validate locally

1. Check out this branch
2. Visit an issue or an MR in the GDK
3. Type some text in the comment field at the bottom of the activity feed
4. Click the "Comment" button. Your comment will be posted.
5. Type another comment and use the dropdown on the split comment button. Choose the first option, "Comment". Your comment will be posted.
6. Type another comment and use the dropdown on the split comment button. Choose the second option, "Start thread". Your comment will be posted as the start of a thread.
7. On an MR, start a review by replying to an existing comment.
8. While in the pending review state...
   1. Compose a new comment at the bottom of the activity feed. Click "Add comment to review". Your comment will be added to the pending review.
   2. Compose another new comment and use the dropdown on the split comment button. Choose the first option, "Comment". Your comment will be added to the pending review.
   3. Compose another new comment and use the dropdown on the split comment button. Choose the second option, "Start thread". Your comment will be added to the pending review.
   4. Compose another new comment and click "Add comment now". Your comment will be immediately added, and will not be pending.
   5. Compose another new comment and click "Add thread now". Your comment will be immediately added as the first comment in a new thread and will not be pending.
9. Finish your review. You will see that all the pending comments are posted, and the comment added in step 8.3 above will be the first comment in a new thread.